On my Europa there is the normal throttle range from 0-100%, then you have to shift the throttle lever sideways through a detent to engage the turbo and use takeoff power (115%). Advice is not to use part of this 100-115% range, you either use full take-off power or just use the normal range.
I normally cruise at 75% 5000rpm for 18-20lph. More patient people cruise at lower power settings.
Seems to be the general consensus is if you are buying new then the 912S is fine for most uses and no point paying extra for the turbo. However the original question was what can be done to uprate the 80hp 912 - can it just be upgraded to 912S spec?